Transcript Exploring the Science of Electricity

Lesson
Exploring the Science of
Electricity
Interest Approach
What is Electricity?
Where does electricity come
from?
How does it get from its source
to where we use it?
Interest Approach
What happens when you walk
across carpet in the winter and
receive a “shock” when you
touch someone or something
metal?
Interest Approach
Static electricity is actually a
transfer of electrons from you to
the object you touch
Student Learning Objectives
Relate electricity to the structure
of elements and atoms.
Explain conductors, insulators,
and semiconductors.
Explain the conventional and
electron theories of electrical
current flow.
Student Learning Objectives
Describe how electricity can be
generated by friction, heat, light,
chemical reactions, and magnetism.
Describe the difference between and
applications of direct current (DC)
and alternating current (AC)
electricity.
Terms
o Alternating current
o Atoms
o Battery
o Conductors
o Conventional
oElectrolyte
oElectromagnet
theory
o Cycle
o Direct current
o Electricity
o Electrodes
oFree electrons
oFriction
oElectron theory
oElectrons
oElements
oFuel cell
oHertz
oInsulators
Terms
o Magnetic induction
o Neutrons
o Photovoltaic effect
o Polarity
o Primary batteries
o Protons
o Secondary
batteries
o Semiconductors
oSolar (photo) cell
oSynchronous
alternator
oThermocouple
oThermopile
oValence
Objective 1:
How does electricity relate
to the structure to of
elements and atoms?
Elements
Elements are substances that
cannot be broken down into simpler
substances using ordinary chemical
methods.
All matter is composed of one or
more elements.
Atoms
Atoms are the smallest units of an
element.
They cannot be divided further
without losing physical and
chemical properties.
Atoms are composed of protons,
neutrons, and electrons.
Protons and Neutrons
Protons have a positive
electrical charge, while
neutrons have no electrical
charge.
Protons and Neutrons
Protons and neutrons make up
the nucleus, or center of each
atom.
Thus the nucleus of each atom
has a positive electrical charge.
Electrons
Electrons have a negative
electrical charge and orbit the
nucleus of an atom in rings or
shells.
Since unlike charges attract each
other, the electrons are held in
orbit by the positively charged
nucleus.
Electrons
The outer ring of electrons is called
the valence ring.
Copper As A Conductor
The element copper, has 29
protons in its nucleus and 29
electrons which orbit its
nucleus.
The electrical charges cancel
each other.
Artwork supplied with permission
of Interstate Publishers, Inc.
Copper as a Conductor
The electrons actually orbit the
nucleus in rings.
–The first or inner ring can hold a
maximum of 2 electrons, the second
ring, a maximum of 8 electrons, the third
ring, a maximum of 18, the fourth ring, a
maximum of 32, the fifth ring, a
maximum of 50, and the sixth, a
maximum of 72.
Copper as a Conductor
Generally, the rings closest to
the nucleus are filled before
additional rings are started.
Thus, copper has 1 electron in
the fourth ring or valence ring
(2 + 8 + 18 + 1).
Copper as a Conductor
The number of electrons in
the valence ring has a key
role in determining the
electrical characteristics of the
element.
Free Electrons moving between
copper atoms
Artwork supplied with
permission of Interstate
Publishers, Inc.
Objective 2:
What are conductors,
insulators, and
semiconductors?
Conductors, Insulators, and
Semiconductors
Conductors, insulators, and
semiconductors are important in
electricity and electronics.
The following will help explain
each:
Conductors
Materials that allow electricity to
flow through them easily.
Copper, aluminum, silver, and
gold are good conductors.
Conductors
Generally, nearly all metals are
good electrical conductors.
Any element or material having
atoms with three or fewer
electrons in its valence ring will
be a conductor.
Insulators
Materials that do not allow
electricity to flow through them
under normal conditions.
Materials such as rubber, plastic,
porcelain, and glass are all good
electrical insulators.
Insulators
Any element or material
composed of atoms having five or
more electrons in the valence ring
will be an insulator.
Insulators are used to confine the
flow of electricity to desired paths.
Semiconductors
Materials that are neither good
conductors nor good insulators.
Semiconductors are manufactured
from elements having atoms with four
electrons in their valence rings.
Silicon and germanium are widely
used in making semi-conducting
materials.
Objective 3:
What are the conventional and
electron theories of electrical
current flow?
Electricity
Electricity is the flow of
electrons from atom to atom in
a conductor.
There are two different
theories to help describe
electricity:
Conventional Theory
Says that electricity is the flow of
positively charged particles through a
conductor.
This is the older theory and was
developed before scientists discovered
the existence of electrons.
With this theory, it was assumed that
current flow in an electrical circuit was
from positive to negative.
The direction of current flow
according to the
conventional theory.
Artwork supplied with permission
of Interstate Publishers, Inc.
Electron Theory
This is the accepted model of
electrical current flow, however,
because of tradition, the
conventional theory is still widely
used.
Either theory may be used as
long as it is used consistently.
Electron Theory
The electron theory says that
electricity is the flow of electrons
through a conductor.
Remember conductors have
three or fewer electrons in their
valence ring.
Electron Theory
When there are so few electrons
in the valence ring they are not
held very tightly.
These free electrons can be
dislodged if sufficient external
force is applied.
Electron Theory
An example of external force is a
battery, which has a positive
terminal and a negative terminal.
The direction of current flow
according to the electron theory.
Artwork supplied with permission
of Interstate Publishers, Inc.
Electron Theory
Since unlike charges attract each other, if
a copper wire were attached to the two
terminals, the negatively charged free
electrons would be dislodged and pushed
by the negative terminal and pulled by the
positive terminal of the battery, causing
the electrons to flow.
This would continue until the battery
discharges.
Objective 4:
How can electricity be generated
by friction, heat, light, chemical
reactions, and magnetism?
How can electricity be generated by
friction, heat, light, chemical reactions,
and magnetism?
An external force must be applied
to cause free electrons to flow
through a conductor.
This force is the production or
generation of electricity, which can
be generated in the following ways:
Friction
Friction is caused when two or
more materials rub against each
other.
Friction
When this happens, some of the
free electrons from one material are
transferred to the other material,
causing one of the materials to have
a positive electrical charge and the
other to have a negative electrical
charge.
Friction
When the negatively charged
material touches a material with
a neutral or positive charge, the
excess electrons will flow to the
second object.
Friction
This can be demonstrated when
you walk across carpet in the
wintertime.
As you walk across the carpet,
the soles of your shoes rub over
the carpet which transfers
electrons from the carpet to your
body.
Friction
When you touch someone else
or a doorknob, the excess
electrons discharge through your
hands resulting in a static
electricity shock.
Friction is not a practical method
of generating electricity.
Thermocouple
Heat or temperature differences
can be used to generate
electricity using a thermocouple.
A thermocouple consists of two
dissimilar metals, such as iron
and nickel, joined together to
form two junctions.
Thermocouple
When heat is applied to one of the
junctions, the difference in
temperature between the junctions
causes free electrons to flow from
the iron wire into the nickel wire and
away from the hot junction toward
the cold junction.
Thermocouple
To increase output, several
thermocouples are often combined
to form a device called a
thermopile.
These are often used in flame
detectors, furnace safety valves,
and in precision heat measurement
devices.
Photovoltaic Effect
Light from the sun can also be used to
generate electricity.
It requires a solar (or photo) cell for
converting the light into electricity
through a process known as the
photovoltaic effect.
Photovoltaic Effect
Certain materials (such as gallium,
silicon, and cadmium sulfide) will
convert light energy into electrical
energy through the photovoltaic
effect.
The solar cell is made of a thin disk
of silicon to which other chemicals
have been added.
Photovoltaic Effect
When light strikes the disk, electrons
move from one side to the other
side.
The electrons move through the
conductors and provide electrical
energy to power the circuit load(s).
–Solar energy is used to power to such
things as calculators and electric fence
controllers.
Chemical
Reactions between certain
chemicals can be used to
produce electricity.
Chemical
First, a battery is made of two or
more chemical cells connected
together.
Each cell is composed of two
dissimilar metal plates called
electrodes.
They are separated from each other
and immersed in an electrolyte.
Chemical
An electrolyte is a chemical
solution that contains positively
and negatively charged atoms
called ions.
Chemical
Using a chemical cell that is
composed of a zinc plate, a
copper plate, and an electrolyte
solution of sulfuric acid and
water as an example, the
acid/water solution reacts with
the zinc plate causing it to lose
positively charged ions.
Chemical
Each positive ion lost leaves
behind two electrons.
Thus, the zinc plate soon takes
on a negative charge.
It then becomes the negative
terminal of the cell.
Chemical
As the positive ions move
through the electrolyte, they
collide with the copper plate.
The positive ions attract free
electrons from the copper plate.
Chemical
Thus, the copper plate soon
takes on a positive charge due
to this loss of electrons and
becomes the positive terminal
of the cell.
Chemical
If we connect a conductor to the
two terminals, electrons will flow
through the conductor from the
zinc plate to the copper plate.
Chemical
This flow will continue until the
difference in charge between the
two plates has dissipated.
Once this happens, the cell will be
discharged.
Chemical
Batteries may be classified as
primary batteries (carbon-zinc,
alkaline, and mercury) which
cannot be recharged or as
secondary batteries (lead-acid
and nickel-cadmium) which can
be recharged.
Chemical
A second chemical reaction to
produce electricity is a fuel cell,
which is similar to a battery, but
different in the way the chemicals
are supplied.
Chemical
In a battery, the chemicals are
built in.
In a fuel cell, the chemicals are
pumped into the cell from an
external source.
Currently, fuel cells are limited to
military and space applications.
Magnetism
The final method of generating
electricity discussed here will be
that of magnetism.
Magnetism
If a bar magnet is suspended freely
from a string, the magnet will turn
until one end points north and the
other end points south.
The end pointing north is said to be
the north pole of the magnet and the
end pointing south the south pole.
Magnetism
An electromagnet may also be
created using electricity.
If electricity flows through an
insulated conductor that is
wound around a metal object,
the metal object will become a
magnet.
Magnetism
Any time a conductor cuts across a
magnetic field or a magnetic field
cuts across a conductor, electricity
will flow in the conductor.
Electricity generated in this manner
is produced through the process of
magnetic induction.
Generation of Electricity
Through Magnetism
Magnetism
This would be a very impractical
method of generating electricity.
Electricity could also be generated
by rotating a magnetic field around
a stationary conductor or by
rotating a conductor inside
stationary magnetic field.
Magnetism
As the rotating conductor cuts
across the lines of magnetism in
the stationary magnetic field,
electrons will be forced to flow
through the conductor.
Magnetism
As the conductor continues to
rotate, the conductor will travel
parallel to the lines of magnetism
and no electrons will flow.
Magnetism
As the conductor continues in its
rotation, it will cut across the lines
of magnetism in the opposite
direction, causing the electrons in
the conductor to flow in the
opposite direction.
The cycle continues as the rotation
continues.
Magnetism
In a commercial electricity
generation power plant, a
synchronous alternator is used to
produce electricity.
This device has a rotating field
winding and a stationary winding.
Magnetism
A small amount of electricity is
supplied to the alternator’s field
windings, which produces a
magnetic field around the
windings (the field windings
become an electromagnet).
Magnetism
As the field windings are turned
(by a turbine driven by an
external power source), the
magnetic field also turns, cutting
across the alternator’s stationary
windings.
Magnetism
Since the conductors are cutting
across a magnetic field,
electricity is induced into the
stationary conductors.
Magnetism
Power plants may burn coal or
use nuclear energy to make
steam from heated water in order
to turn the turbine, or they may
use the kinetic energy of falling
water to turn the turbine.
Objective 5:
What is the difference
between direct current and
alternating current and what
are some applications of
each?
AC/DC
Electricity may be classified as
direct current or alternating
current depending on the pattern
of flow of the electrons in the
circuit.
Direct Current
In direct current or DC electricity,
the electrons flow in only one
direction.
Direct Current
Since electrons flow from the
negative terminal to the positive
terminal in an electrical circuit,
sources of DC electricity must
have a fixed polarity.
Direct Current
This means one specific terminal
is always negative while the
other is always positive.
Thermocouples and thermopiles,
solar cells, batteries and fuel
cells all produce DC electricity.
Direct Current
A DC generator may also be
used to produce DC electricity
through magnetism.
Alternating Current
With alternating current or AC,
electricity flows first in one
direction, stops, reverses and
flows in the opposite direction.
Alternating Current
Once this occurs, the electricity
is said to have completed one
cycle.
–AC electricity is the type
generated by electric power
plants and is what is used in
homes, businesses, and other
locations.
Alternating Current
One cycle of electrical flow is
produced with each revolution of the
plant’s synchronous alternator.
–In the U.S., they turn at a speed of
60 revolutions per second.
–Therefore, current generated in the
U.S. completes 60 cycles per
second.
Alternating Current
The term hertz (Hz) represents
one cycle per second, so in the
U.S. our electricity is generated
and delivered at 60 Hz.
In several other countries, 50 Hz
AC electricity is the standard.
Review / Summary
1. Relate electricity to the
structure of elements and atoms.
Review / Summary
2. Explain conductors, insulators,
and semiconductors.
Review / Summary
3. Explain the conventional and
electron theories of electrical
current flow.
Review / Summary
4. Describe how electricity can
be generated by friction, heat,
light, chemical reactions, and
magnetism.
Review / Summary
5. Describe the difference
between and applications of
direct current (DC) and
alternating current (AC)
electricity.